1. Field of the Invention
This invention relates to x-ray imaging systems. In a primary application the invention relates to obtaining images representing specific materials in the body.
2. Description of Prior Art
Present x-ray imaging systems record the transmission of objects, such as body tissue, to a broad energy spectrum. This spectrum is generated by the "bremstrahlung" or braking radiations caused by an electron beam when it strikes a metallic target. This broad energy spectrum does not allow the radiologist to distinguish between various materials. Thus the total information in a conventional radiograph is due to non-selective absorption of materials.
It would be highly desirable to be capable of deriving information indicating specific regions of the x-ray spectrum. These could be used to form images representing specific materials of the body.
Unfortunately, band-pass filters which transmit specific regions of the x-ray spectrum do not exist. Only relatively subtle filtering can be accomplished in the x-ray region because of the nature of the absorption characteristics of materials.
There have been a number of earlier, relatively impractical efforts at delineating selective energy spectrum information using x-rays. These include B. Jacobson and R.S. Mackay, "Advances in Biological and Medical Physics," Academic Press, Vol. VI, (1958); R.S. Mackay, IRE Transactions on Medical Electronics, Vol. ME 7, p. 77 (1960); and B. Jacobson, American Journal of Roent., Vol. 91, p. 202 (1964). These have involved the use of mechanically scanned, monochromatic x-ray sources to obtain transmission information at specific wavelengths. Mechanical analog computers were used to find the thicknesses of the various body materials at each point by moving wedge shaped structures into the x-ray beam until the required output was obtained. Many of these studies produced interesting diagnostic results including images of specific body materials including bone, soft tissue and the natural iodine in the thyroid. However, the limitations of the relatively low energy monochromatic sources, the relatively slow mechanical scanning and the low resolution of the resultant images made these systems unsuitable for clinical diagnostic radiology.
The use of gratings to encode spectral information is well-known in the visible spectrum. Here the principle incentive is to avoid the registration problem that would otherwise ensue using three separate films or television cameras for the red, green and blue images. The problem is not, as in the x-ray spectrum, the availability of suitable spectral filters for the various spectral components.
Some examples of such encoding and decoding systems for color renditions in the visible spectrum are illustrated in U.S. Pat. No. 2,050,417 granted to C. Bocca, U.S. Pat. No. 3,504,606 granted to A. Macovski and U.S. Pat. No. 3,378,633 granted to A. Macovski.